Many studies has been made in the prior art of various configurations of waveguide and slab-waveguide CO.sub.2 lasers due to their compact size, high power per volume, and low manufacturing cost. A waveguide laser is different from a conventional laser because, in a waveguide laser, light does not follow the laws of free-space propagation over some (or all) of the propagation path inside a waveguide resonator structure, i.e., some structure within the laser acts to confine and guide the light. In more mathematical terms, one uses Fresnel number, defined as a2/(.lambda.L), to characterize a waveguide resonator; where a is the half-width of an exposed mirror surface, .lambda. is the wavelength of light inside the resonator, and L is the resonator length. The Fresnel number measures the importance of diffraction effects and a low number indicates strong diffraction. A waveguide resonator has a Fresnel number on the order of unity or less and, as a result, spatial modes, frequency spectra, and losses are not adequately described by laws governing free-space resonators.
A slab-waveguide laser is different from a standard waveguide laser because a slab-waveguide laser does not confine propagation light fully, but does so in one dimension only. For example, consider a Cartesian coordinate system (x,y,z) with light propagating in the z-direction. A rectangular waveguide laser would confine light in the x- and the y-direction, whereas, a slab-waveguide laser would confine light in the x- or the y-direction (but not both), with the unconfined direction obeying normal free-space propagation theory.
Slab-waveguide lasers are disclosed in many U.S. patents, notably U.S. Pat. No. 4,719,639 (Tulip) issued Jan. 12, 1988; U.S. Pat. No. 4,939,738 (Opower) issued Jul. 3, 1990; and U.S. Pat. No. 5,048,048 (Nishimae) issued Sep. 10, 1991. The Tulip patent discloses an RF-excited slab-waveguide laser which uses a positive-branch unstable resonator for the unconfined direction and a stable waveguide resonator for the guided direction. The Opower patent discloses an RF-excited slab-waveguide laser which uses a positive-branch unstable resonator for the unconfined direction and which also uses astigmatism corrective optics, which astigmatism corrective optics are incorporated as part of the laser structure. Astigmatism is a by-product of the slab geometry (except in certain specially chosen resonator designs). As a result, astigmatism correction is usually required, although it need not be part of the laser housing. The Nishimae patent discloses a microwave-excited slab-waveguide laser which uses spherical mirrors to define a negative-branch unstable resonator in the unconfined direction. The negative-branch resonator exhibits an internal focus typically found within the active region of the laser which, if the intensities are high enough, causes loss of efficiency and possibly even optical breakdown of the gain medium. However, as is well-known in the art, negative-branch unstable resonators are much less alignment sensitive than their positive-branch counterparts. Further, as pointed out in the Nishimae patent, the slab-waveguide produces a line focus, not a point focus, within the medium; hence intensities are reduced somewhat. Therefore, depending on power levels and size of the device to be constructed, either positive-branch or negative-branch designs may prove optimal.
Refinements to the basic slab lasers disclosed in the above-identified patents have been disclosed in further U.S. patents, specifically: U.S. Pat. No. 5,123,028 (Hobart, et al.) issued Jun. 16 1992; U.S. Pat. No. 5,131,003 (Mefferd) issued Jul. 14 1992; U.S. Pat. No. 5,131,004 (Dallarosa, et al.) issued Jul. 14, 1992; U.S. Pat. No. 5,140,606 (Yarborough, et al.) issued Aug. 18, 1992; and U.S. Pat. No. 5,155,739 (Mefferd) issued Oct. 13, 1992. The Hobart et al. patent discloses the use of spherical mirrors in a negative-branch geometry, which spherical mirrors are placed a prescribed distance from the slabs so that the radius of curvature of light in the confined (waveguide) direction at the mirror surface approximately matches the radius of curvature of the mirror. The Mefferd '003 patent discloses the use of a support structure for electrodes that allows for thermal expansion during operation of the laser. The Dallarosa et al. patent discloses a method of pre-ionizing a CO.sub.2 gas mixture to help stabilize the discharge, especially when the laser is run in a low duty-cycle pulsed mode. The Yarborough et al. patent discloses a mechanism for holding electrodes a required distance apart while minimizing constrictions on gas flow within the laser housing. Lastly, the Mefferd '739 patent discloses a mechanism for holding adjustable mirrors in a low-pressure, sealed-off environment.
Most slab-waveguide CO.sub.2 lasers exhibit switching between rotational transitions of the CO.sub.2 molecule that occur within a specific vibrational transition, which switching produces relatively small variations in wavelength. This switching between rotational transitions within a specific vibrational transition is common to most CO.sub.2 lasers and has become known in the art as "line-hopping." For medical applications, the small variations in wavelength caused by line-hopping do not present a problem and can usually be ignored.
However, we have discovered a phenomenon which is unknown in the art. In particular, we have discovered that, unless special care is given to the design of both the slab-waveguide and the resonator optics, a slab-waveguide CO.sub.2 laser is capable of emitting light over a wide range of wavelengths, which wavelengths arise from two different vibrational transitions of the CO.sub.2 molecule. These wavelength regions are centered approximately at 10.6 .mu.m and 9.4 .mu.m, respectively. The transition producing 10.6 .mu.m radiation has the highest gain of any transition of the CO.sub.2 molecule and, therefore, nearly all conventional CO.sub.2 lasers operate at 10.6 .mu.m without the need to take precautions to avoid laser light from occurring at 9.4 .mu.m. However, using slab geometries and standard broadband optics as described in the prior art, we have discovered that a slab-waveguide CO.sub.2 laser can switch erratically from one vibrational transition to the other and back again.
The lack of understanding of this problem in the prior art occurs for two reasons: lack of continuous monitoring of the output beam and fortuitous waveguide suppression. Wavelength switching which occurs in lasers fabricated according to prior art teachings are understood from the following. FIG. 9 shows, in pictorial form, a configuration used to measure output power from conventional slab-waveguide laser 100 utilizing ZnSe beamsplitter 10. As shown in FIG. 9, output beam 110 from slab-waveguide laser 100 is split into reflected beam 120 and transmitted beam 130. Reflected beam 120 is detected by detector 11 and transmitted beam 130 is detected by detector 12, reflected beam 120 and transmitted beam 130 are monitored simultaneously. FIG. 9A shows a measured plot of ZnSe beamsplitter 10 characteristics. As seen from FIG. 9A, ZnSe beamsplitter 10 has an approximately 1% measured reflectivity at 10.6 .mu.m and an approximately 4% measured reflectivity at 9.4 .mu.m. This is to be contrasted with a variation due to line hopping which would produce a variation of about 10% in the transmitted and reflected signals.
FIG. 10 shows a plot of measured output power of conventional slab-waveguide CO.sub.2 laser 100 vs. time taken simultaneously from detector 12 monitoring transmitted beam 130 and detector 11 monitoring reflected beam 120. As seen in FIG. 10, reflected beam 120 experiences very wide fluctuations in relative power, while transmitted beam 130 is very nearly constant, except for the .sup..about. 10% fluctuation expected from normal "line-hopping."
FIG. 11 is a plot of measured output power of conventional slab-waveguide CO.sub.2 laser 100 vs. time obtained using the configuration illustrated in FIG. 9. However, in generating FIG. 11, conventional slab-waveguide CO.sub.2 laser 100 was run so as to mimic actual surgical use. That is, fifteen (15) laser bursts, each 45 seconds long and separated by two (2) minutes off-time, were recorded by a digital oscilloscope and the high-low envelope of the bursts is shown in FIG. 11. As can be seen in FIG. 11, in agreement with FIG. 10, the envelope of the reflected beam is much greater than that of the transmitted beam. This confirms that conventional slab-waveguide laser 100 switched wavelength over a wider range than could be accounted for by line-hopping and that the change in reflectivity of beamsplitter 10 for the different wavelengths caused wide fluctuations in reflected beam 120.
FIG. 12 shows, in pictorial form, a configuration used to confirm that the above-described wavelength fluctuations were due to switching between vibrational bands of the slab-waveguide CO.sub.2 laser. As shown FIG. 12, output beam 110 from slab-waveguide laser 100 impinges upon ZnSe beamsplitter 10, which beamsplitter has a reflectivity of 10% at 10.6 .mu.m and a reflectivity of 12% at 9.4 .mu.m. Reflected beam 120 is spectrally separated by diffraction grating 140 (-- order, near-Littrow) into spectral bands corresponding to the 10.6 .mu.m band and the 9.4 .mu.m band detected by detectors 150 and 160, respectively. FIG. 13 is a simultaneous plot of the 10.6 .mu.m band and the 9.4 .mu.m band in output beam 110 from detectors 150 and 160, respectively. As can be seen from FIG. 13, slab-waveguide laser 100 is periodically switching wavelengths. It can also be seen that the sum of both components is roughly constant, in agreement with the transmitted portions of the output beam presented in FIGS. 10 and 11.
Two facts become clear from the above. First, wavelength in the output beam of a conventional slab-waveguide CO.sub.2 laser changes dramatically over time and, second, total laser output remains nearly constant over time. Thus, since prior art apparatus typically do not continuously monitor the laser output beam, wavelength switching has gone unnoticed. A further reason that wavelength switching has not been detected in the prior art is that it is possible, as a fortuitous matter, to choose a waveguide design that helps suppress one of the wavelength regions. However, even though this is possible, we have discovered that the selectivity of a waveguide itself is generally quite low i.e., the losses are comparable for 9.4 .mu.m and 10.6 .mu.m. For that reason, we have discovered that one cannot depend on the selectivity of a waveguide structure alone to reliably suppress unwanted wavelength switching.
The large wavelength shift produced by changing vibrational transitions causes a problem for CO.sub.2 lasers used in medicine due to the high water absorption of the CO.sub.2 wavelength. Since the primary constituent of most tissue is water, this absorption means that CO.sub.2 laser energy is applied efficiently and, equally as important, locally to target tissue. The measure of locality of a laser-tissue interaction depends on the size of the focused beam and the penetration depth of the beam, which penetration depth is governed mainly by the water absorption coefficient. Referring to tabulated values of absorption coefficients for water (for example, "Infrared Optical properties of Water and Ice Spheres", W. M. Irvine and J. B. Pollack, Icarus 8, [1968], p. 324), the absorption coefficient is 720 cm-1 at 10.5 .mu.m and drops to 410 cm-1 at 9.5 .mu.m. Although exact values are not tabulated, the coefficients vary smoothly with wavelength and the values at 10.5 .mu.m and 9.5 .mu.m are quite close to those at 10.6 .mu.m and 9.4 .mu.m. Therefore, the characteristic penetration depth (defined as the point where laser intensity is .sup..about. 63% of its initial value) is .sup..about. 13.9 .mu.m at 10.6 .mu.m and it increases to .sup..about. 24.4 .mu.m at 9.4 .mu.m; a 76% variation. As a result, for consistent treatment results, especially in delicate surgery, it is imperative to keep the laser operating within a given vibrational transition.
In light of the above, we have discovered that there is a need to prevent switching between vibrational transitions in a slab-waveguide CO.sub.2 laser and, as a result, there is a need in the art for a slab-waveguide CO.sub.2 laser that is stable and that operates within a given vibrational transition of the CO.sub.2 molecule.